DESCRIPTION: The long-term goal of this proposal is to study the dynamics of T cell activation in humans during primary and secondary responses to a viral infection. To quantitate human anti-viral T cell responses and to analyze the differentiation of effector and memory T cells from na&iuml;ve T cells in vivo, the PI selected yellow fever virus as their viral infection model. The goals of this proposal are two folds. First, to initiate studies to get glimpse of the magnitude and kinetics of the primary and memory T cell responses to YFV in humans; and second, to identify T cell epitopes of YFV so that a more precise analysis of the immune response can be done in subsequent studies. In addition, the proposed studies allow them to do an experiment addressing the important issue of "bystander activation" during viral infection.